Amplifier



Oct. 10, 1933.`

R. B. DOME urLIFnan Piled March 1, 1933 Fig?) X D A l Q l NE o k l 1 c i B Inventor: 20000- FREQUENCY v RObePt B. DOTTle,

H is Attorney.

Patented Oct. 10, 1933 AMPLIFIER Robert B. Dome, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application March 1, 1933. Serial No. 659,199

4 Claims.

My invention relates to amplifiers and more particularly to impedanceor resistance-capacitance coupled amplifiers for use over a Wide range of audible and super-audible frequencies.

In transmitters and receivers such as are used for television purposes, it is necessary to provide an amplifier having a substantially flat amplification characteristic lfor amplifying the frequency impulses which produce the television image. Such frequency impulses may range from an audio frequency of 20 cycles to a super-audible frequency of 250,000 cycles or more. It has been found that the ordinary resistance-capacitance coupled amplifier amplifies both audible and super-audible frequencies, and this amplifier would be suitable for this purpose except for the fact that the amplification characteristic begins to droop appreciably in the neighborhood of 20,000. cycles and this drooping becomes more prominent as the frequency increases. To obtain the proper amplification of the higher frequencies Whichextend well into the radio frequency range,

several stages of amplification would be necessary and the lower frequencies would tend to overload the amplifier. This drooping characteristic is largely due to the inter-electrode capacities present within the electron discharge amplifying device.

In accordance with my invention, I provide a resistance-capacitance coupled amplifier with means for neutralizing the effects of the gridto-anode capacity, particularly at the higher frequencies at which the amplifier is operated. This arrangement prevents the amplification characteristic from drooping, due to the elimination of the effect of the inter-electrode capacities of the electron discharge device.

My invention has for one of its objects to pro- Vide a resistance-capacitance coupled amplifier with means for neutralizing the grid-to-anode capacity at the higher frequencies.

A further object of my invention is to provide a resistance-capacitance coupled amplifier with an arrangement for maintaining the gridto-anode impedance at a high Value for the low frequencies Vand increasing this impedance as the frequency increases throughout the operating range of the amplifier.

The novel features which I believe to be characteristic of my invention are set forth in the appended claims. My invention itself, however, both as to its organization and method of operation, together with other objects and advantages thereof, will be better understood by referring to the following description taken in connection with the accompanying drawing in which 7 Fig. 1 illustrates one form of the invention; Fig. 2 illustrates a preferred embodiment of my invention, and Fig. 3 illustrates the operating characteristic of my invention.

low audible frequencies and relatively high radio frequencies. This source may for example comprise a television machine, or one of the earlier stages of ran amplifier for television currents.

by a source of potential 2 connected to the cathode and the Vnormal Vgrid coupling resistor 3. Anode current is supplied to the electron discharge device 1 from a source of potential 4 through the anode coupling resistor 5. The anode coupling capacitor 6 permits the amplifier to be coupled to a succeeding similar stage of amplification as shown or any suitable utilization device. The amplifier just described constitutes the 'usual form of an impedanceor/ resistance-capacitance coupled amplifier. A source of oscillations 7 is connected to the input circuit by connections to the grid and the cathode of the discharge device 1.

In accordance with my invention, an inductor 8 is connected in the input circuit between the grid coupling resistor 3 and the grid or control element of the electron discharge device l. This inductor 8 is so constructed and arranged that at low frequencies it has substantially no impedance, but at the higher frequencies and throughoutV the remainder of the operating range of the amplifier it increases in impedance with the increase in frequency of the oscillations to be amplified. A second inductor 9 is inductively coupled to the inductor 8 and in this particular embodiment is connected at one extremity to the inductor 8. The inductors 8 and 9 may be sections of a continuous inductor as disclosed in Fig. 1, with the midpoint connected to the grid coupling resistor 3. The other extremity of the inductor or inductor section 9 is connected through a variable capacitor 10 to the cathode of the electron discharge device. The capacity of this capacitor is normally adjusted to a value equal to the normal grid-to-cathode capacity of the electron discharge device 1. A neutralizing or balancing capacitor 11 is connected between the anode of ..60 Referring to Fig. 1 of the drawing, I have;

,65 quencies extending over a. range including both ,.70 The control element may be biased negatively'- the discharge device and a lpoint between the capacitor 10 and the inductor 9. The capacitor 11 is adjusted to a value equivalent to the normal grid-to-anode capacity of the electron discharge device. The inductors 8 and 9, as now arranged, each has the same capacity value across it, as the inductor 8 has across it the grid-to-cathode capacity of the discharge device while the'inductor 9 has across it the capacitor 10 which is adjusted to the same capacity value as the gridto-cathode capacity. Y

The operation of this amplifier at the lower frequencies is that of the ordinary resistance' capacitance coupled amplifier, as any voltage impressed across the input circuit does not produce any appreciable drop across the inductor 8. However, as the amplifier operates at the higher frequencies, a portion of the voltage produced by the amplifying action of device 1 across anode resistor 5 is coupled back through the capacitor 11 to produce magnetic flux in the inductorg. The flux of linductor 9 linksY with inductor 8 so .that Va potential is induced therein which is applied to the grid of the discharge device l. The

voltage impressed upon this grid increases with the frequency of the amplified currents and is yoppositein phase to the voltage impressed therel on by the coupling of the output voltage across the resistors through the grid-to-anode capacity of the discharge device, At the highest frequenciesamplined, the voltage impressed upon the grid is equal to the voltage impressed thereon due to the grid-to-anode capacity. The impedance froml gridfto cathode in this arrangement may be expressed by the following equation:

"Where, R1 is'the resistance of the coupling requency. It is therefore apparent from this equay tion that as ythe higher frequency ranges are .appwnhedby the amplifier the inductive re- -stance of L becomes appreciable so that the impedance Z will increase with the frequency.

Referring to Fig. 2, I have shown therein an amplifier of the resistanceapacitance coupled type which is similar to that shown in Fig. 1.

- like parto in Fil. 2 are given the same reference numerals as in Fig, 1. `It will be noted, however,

that the inductors 8 and 9 in Fig. 2 are separate 4 and distinct and that the upper extremity of the inductor 9 is now connected at a point between the coupling resistor 3 and the grid biasing potential 2, thus `avoiding losses through the resistor 3. and that the side of the capacitor 10 which formerly was connected directly to the cathode is now connected to this same point between the resistor yand biasing source of potential. This arrangement has the advantage that the effect of neutralization of grid-to-,anode capacity lbecomes eifective at lower frequencies than in the circuit in Fig. 1 `while still maintaining the same value of inductors and coupling resistors. Thus in this arrangement the voltage drop across 8 is appreciable at a somewhat lower frequency since none of the voltage present across resistor 5 which is coupled throughV capacitor l1 to the inductor 9 is impressed upon the resistor 3. This improved resistance-capacitance coupled amplifier, therefore, permits a higher voltage to be induced in the inductor 8 at the lower frequencies,

quencies Where the induced voltage is equal to 'modifications may be made in the circuit arrangements and inthe instrumentalities emplification characteristic of a resistance-capacitance coupled amplifier which begins to droop vappreciably in the neighborhood of 20,000 cycles. The curve shows amplification in decibels plotted against frequency with the line X at the highest frequency to be amplified. The improvement in the characteristic due to the use of an inductor 8 in the input circuit is shown in curve AC, and this is primarily due to the fact that the inductor tends to maintain constant the impedance of the input circuit. The curve AD shows the improvement obtained by the neutralization of the grid-to-anode capacity. Curve AE shows the amplification characteristic of an amplifier such as shown in Fig. 2 wherein the inductors 8 and 9 and associated circuits cooperate to maintain the amplification constant over the entire frequency range to the' highest frequency to bevamplied.

YThe embodiments of the invention illustrated and described herein have been selected for theA purposeV of clearly setting forth the principles in! volved. It will, of course, be understood that I rdo not wish to be limited thereto, since it is apparent that `'the principles herein disclosed are susceptible of numerous other applications, and

ployed without departing from the spirit and scope of my invention as set forth in the appended claims.

yWhat I claim as new and desire to secure byl() Letters Patent of the United States is:

Y 1. The combination, in an impedance-capacitance coupled amplifier, of an electron discharge device having a cathode, an anode and a grid Y element, an' input circuit forsad device includ- 115 ing .a coupling impedance connected toV said cathode and said grid element, a source of .Oscil- 'lations connected to said input circuit for amplification, said oscillations extending over va fre-..,

quency range from low audio frequencies to rela-" tively high radio frequencies, an output circuit for said device including a coupling impedance connected between said cathode and anode, an inductcr 'connected in said input circuit betweenU said grid and said coupling impedance, said inductor having substantially no impedance at audio frequencies and having increasing impedance with increasing frequency in the higher frequency position of the operating frequency.

range, and means .connected to said anode and" said input circuit including an inductor arranged in inductive relation to Asaid first inductor for impressing upon said grid a voltage lincreasing in value withV the increasing frequency of the ampli-,

ed currents and opposite in phase to the voltage impressed upon said grid through the inherent capacity between said anode and said grid element.

2. The combination in a resistanceecapacitance coupled amplifier comprising an electron discharge device having Aan anode, a cathode and a grid, an input circuit including a coupling resistor connected between said cathode and grid, an output circuit including a coupling resistor- .connected between said'cathode and anode and va coupling capacitor connected to said anode, a source of oscillations connected to said input circuit for amplification, said .oscillations having frequencies extendingv over a frequency range from low audio frequencies to relatively high radio frequencies, said amplifier having an amplification characteristic which is drooping in the higher frequency range, of means connected in said input circuit for increasing the input impedance thereof with the increasing frequency in said higher frequency range, and means connected between said input circuit and said anode for impressing upon said grid a voltage increasing in value with the increasing frequency of the higher frequencies of the oscillations and opposite in phase to the voltage impressed upon said grid through the inherent capacity between said anode and said grid whereby said means cooperate to prevent said amplification characteristic from drooping.

3. The combination, in a resistance-capacitance coupled amplifier comprising an electron discharge device having an anode, a cathode and a grid, an input circuit including a coupling resistor connected between said cathode and grid, an output circuit including a coupling resistor connected between said cathode and anode and a coupling capacitor connected to said anode, a source of oscillations connected to said input circuit for amplification, said oscillations having frequencies extending over a frequency range from low audio frequencies to relatively high radio frequencies, said amplifier having an amplification characteristic which is drooping in the higher frequency range, of an inductor connected in said input circuit betwen said grid and said coupling resistor, said inductor being arranged to produce substantially no effect at low audio frequencies and to produce an impedance increasing with the increase in frequency of said oscillations through the remainder of the operating range of said amplifier, and means to impress upon said grid a voltage increasing in value with the increasing frequency in said higher frequency range of said oscillations and opposite in phase to the voltage impressed upon said grid through the inherent capacity between said anode and said grid whereby said amplification characteristic is maintained substantially constant.

4. In a resistance-capacitance coupled amplifier normally having a drooping amplification characteristic, an electron discharge device having a cathode, an anode and a grid, an input circuit comprising a grid coupling resistor connected to said cathode and an inductor connected between said resistor and said grid, a source of oscillations connected to said input circuit for amplification, said oscillations having frequencies extending over a frequency range including both low audible frequencies and relatively high radio frequencies, an output circuit comprising a coupling resistor connected between said cathode and said anode and a coupling capacitor connected to said anode, a second inductor arranged in inductive relation to said first inductor and connected in series with a capacitor having a value equal to the inherent grid-to-anode capacity, said latter capacitor and said second inductor being connected between said cathode and said anode, and a capacitor having a value equal to the cathode-to-grid capacity connected across said second inductor, said inductors and capacitors cooperating to prevent the amplification characteristic from drooplng.

ROBERT B. DOME. 

